The portable hand-held circular saw is a low-cost tool that is broadly used for rough carpentry.
Unfortunately this tool, even when guided by a straight-edge clamped to the workpiece, does not make clean, unsplintered cuts. Some splintering of the cut edge of the top surface of the workpiece is inevitable, even with a fine-toothed saw blade.
The cutting action of the blade is such that the teeth at the bottom edge of the blade move away from the operator and upward to the soleplate. The teeth thus tend to lift the workpiece so that it is necessary to hold the soleplate firmly against the workpiece to avoid "chattering" or binding, which can be dangerous.
The hole in the soleplate, through which the blade projects, is wide and does not support the wood fibers against the splintering forces of the saw teeth. Cutting the workpiece with the show surface facing down is of some help but the hand-held portable circular saw is not normally associated with fine carpentry, such as the cutting of door and window trim or making picture frames.
The present invention overcomes this inherent difficulty and indeed allows a well-made portable circular saw instantaneously to be converted into a machine which can make cross-cut and 45.degree. miter cuts which are as good as those of a radial-arm-saw. Indeed, with care, it can provide cuts of cabinet-maker quality.
Before describing this invention it is necessary to explain the key characteristics of the various forms of the circular saw that are available to the craftsman. The most broadly useful tool currently available is the table-saw. With this tool the circular blade is driven by an arbor which is located below the plane of the table. The blade projects through the table from the underside, and the height and angle of the blade projection are separately adjustable.
The table-saw permits precise miter cuts and square cuts to be made by the operator holding the workpiece against a variable-angle travelling miter-jig, which is restrained to travel in a direction parallel to the flat side of the blade. The workpiece is fed into the front edge of the blade, i.e. the edge nearest to the operator. The direction of rotation is such that the teeth rise from beneath the table at the back of the blade, move toward the operator above the table and then move downwards at the front of the blade to disappear beneath the table again.
The downward cutting action of the teeth tends to hold the workpiece firmly to the table. There is no "chattering". The blade gently resists the workpiece being pushed into it. The front teeth provide a clean cut without splintering of the top surface of the workpiece although the bottom edge may be more or less splintered depending on the size and shape of the teeth. If the workpiece is held firmly against a well-designed miter-jig, if its line of travel is exactly parallel to the plane of the blade, if the "set" of the teeth is uniformly wider than the thickness of the blade, then the rear teeth will travel in the cut made by the front teeth and will not cause top-edge splintering.
This cutting action makes the table-saw, of all circular saws, particularly well-suited for "ripping" operations, i.e. cutting a very long piece of wood in the length direction. In the ripping mode a straight-edge called a "fence" is clamped parallel to the blade at the desired distance from the blade. The workpiece is caused to slide firmly along the fence into the blade. If the fence is perfectly parallel to the blade, if the work is firmly held against the fence until the cut is completed, and if the workpiece does not warp immediately after it is cut, then only the front teeth will contact the workpiece. If for some reason the workpiece becomes misaligned during cutting so that the rear teeth contact the workpiece, then the workpiece may be lifted up by the rear teeth and hurled toward the operator. This is called "kickback" and it can be dangerous. For this reason some craftsmen prefer to use a band-saw, which is not a circular saw at all, for ripping operations.
The radial-arm-saw was developed to accommodate the fact that it is not easy to move long lengths of wood in a cross-cut mode past a table-saw blade, because of the mass and the friction of the wood against the table and other support. With a radial-arm-saw the saw travels instead of the workpiece. It travels, supported by a carriage, along a radial arm which is supported at one end by a vertical post around which the arm can pivot. The radial arm can also be raised and lowered on the vertical post.
The blade is mounted directly on the motor shaft or on a stub shaft geared directly to the motor. The blade and motor assembly can be locked into a variety of positions on the carriage thus permitting bevel and even horizontal cuts. The radial-arm-saw is a sophisticated versatile machine.
Unlike the table-saw the axis of rotation of the blade lies above the plane of the table and the blade only penetrates the table less than 1/8". This changes the cutting action fundamentally.
In the cross-cut mode the saw blade is normally parked away from the operator behind a straight, low, wooden "fence" which is at right angles to the plane of the blade. The workpiece is held against the front of the fence on the table, and the saw blade is pulled on its carriage through a saw-cut in the fence and through the workpiece toward the operator. The edge of the blade nearest to the operator is the cutting edge and its direction of rotation is always downwards towards the table and then backwards towards the fence. The cutting forces therefore hold the workpiece firmly to the table and to the fence. Since the workpiece is stationary there is little chance of the rear teeth catching and lifting the workpiece.
There is no splintering of the top edge of the cut, and very little splintering of the bottom edge of the cut, because the bottom surface of the workpiece is supported by the wooden table right up to the blade edge. When cross or miter cutting, then, the radial-arm-saw can make very clean unsplintered cuts suitable for the finest cabinetry work.
If instead of pulling the saw blade from its parking-place behind the fence and through the workpiece, the saw is pushed from a position at the end of the radial arm nearest to the operator through the workpiece, then an entirely different cutting action occurs. Since the far edge of the saw blade, which is rising, now does the cutting, it tends to lift the workpiece up from the table, and since there is no support for the upper surface of the workpiece, substantial splintering occurs at the upper edge of the cut.
The only disadvantage of pulling the saw through the workpiece is that the blade tends to pull itself into the work. Unless the travel is firmly restrained, the saw-blade may "climb into the workpiece" instead of cutting it cleanly and the saw may jam. This tendency is easily overcome by even an unskilled operator.
The usual arrangement of a radial-arm-saw is to have the fence mounted a sufficient distance in front of the vertical post to permit the saw to be parked behind the fence. When so mounted, a typical radial-arm-saw is capable of cross-cutting a 13" board. But in a left-hand 45.degree. miter-setting it can cut only a 3" wide board. The width may be increased by moving the fence backwards, but this tends to be a time consuming operation. Also the blade has to be raised and lowered for each change of miter. So the radial-arm-saw copes with miter changes with much less efficiency than its sophisticated construction seems to indicate.
Recently miter-boards have become available; these are intended to guide the travel of a portable circular saw to permit accurate miter cuts. In one such typical device, the soleplate of the saw sits on parallel step-tracks about 21/2" above a table. The workpiece is held against a variable-angle miter fence and the saw blade is pushed through the work. The teeth tend to lift the workpiece and to splinter the upper edge of the cut. In a piece of trim this is the show surface, so the miter-board is just not good enough for fine carpentry.
Moreover, the portable saw just sits on the step-tracks. It is not held down by them. So the portable saw cannot be pulled backwards on the step-tracks because it would lift in a dangerous manner. Also when a number of 45.degree. and 90.degree. cuts must be made in a mixed sequence it becomes a nuisance reindexing the miter angle and rechecking its accuracy each time.